1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet in a predetermined direction, and to an image recording apparatus having the same.
The sheet described herein may be a recording sheet on which an image is recorded, a document sheet including an image to be read thereon, or the like.
Further, the image recording system described herein may be a copying machine, a printing apparatus, a word processor, an electronic typewriter, a facsimile system, or the like.
2. Related Background Art
FIG. 9 shows an example of an ink jet recording apparatus having a sheet conveying apparatus which has been previously proposed by the same applicant as the present application (but, not known yet).
In the ink jet recording apparatus shown in FIG. 9, a recording sheet 11 such as a paper, a film, a cloth or the like is fed by a feed roller 10 until a leading edge of the recording sheet is pinched by a nip between a conveying roller 1 and an intermediate roller 2. Then after the images included in one line corresponding to a lengthwise width of an array of recording elements, for example nozzles in the ink jet printer, are recorded on the recording sheet by scanning a head unit 7, the recording sheet is conveyed only by the conveying roller 1 by one pitch corresponding to the recording width. A motor M is provided for driving the conveying roller 1 through a belt transmission mechanism M and for driving the feed roller 10 through an appropriate transmission mechanism (not shown). While the recording sheet 11 is conveyed, it is closely contacted to a platen 8 by sucking air by an appropriate suction blower (not shown) through a plurality of small holes 9 formed in the platen.
Further, a biasing roller section 101 is provided for cooperating with the feed roller 10 to pinch the recording sheet 11 therebetween. The biasing roller section is divided into three rollers 101a, 101b and 101c to conform the width of various recording sheets. More particularly, the roller section is divided at positions corresponding to the longer widths of a A4 size sheet and of a A3 size sheet from one of the reference side edges at which one side edge of the sheet to be conveyed is registered.
Another biasing roller section 3 comprising three rollers 3a to 3c divided in the same manner as the above-mentioned biasing roller section 101 is also provided for urging the intermediate roller 2 against the conveying roller 1 to pinch the recording sheet 11 therebetween. The divided rollers 3a to 3c are rotatably mounted on corresponding arms 6a-6c pivotably mounted on a common shaft 5, respectively. Biasing springs 4a to 4c arranged between biasing bars 13a to 13c fixed to the corresponding rockable arms 6a to 6cand corresponding fixed plates 12a to 12c are provided for biasing the rollers 3a to 3c toward the intermediate roller 2.
As described in the Japanese Patent Laid-Open No. 63-8142 (which is published on Jan. 13, 1988 and corresponding to Japanese Patent Application No. 61-146094 filed on June 24, 1986) submitted by the same applicant as the present application, the intermediate roller 2 having a uniform diameter along an axial direction thereof and the biasing roller means 3 divided at a predetermined positions are provided for the purpose of pinching the recording sheet 11 with proper friction force. And, the biasing roller means 3 can be adjusted to obtain more uniform distribution of the biasing force; thus, when a small-sized recording sheet is conveyed along a conveying path with its one side edge being registered with one of the reference side edges, the skewing of the recording sheet being conveyed can be minimized or prevented effectively. Further, if there is no intermediate roller and segment areas for segmenting sections of the platen to be sucked in accordance with the width of the recording sheet and coincident with positions where the biasing rollers are not present, or positions between the biasing rollers, the recording sheet after being printed may be partly swelled at divided portions due to the ink and the like sticked thereon, thus floating the sheet portion above the platen. Accordingly, the intermediate roller is also adapted to prevent such floating of the recording sheet.
Therefore, in order to accurately convey any recording sheet from a large-sized sheet to a small-sized sheet without skew thereof, it is effective to provide the intermediate roller having the uniform diameter along the axial direction thereof and the biasing roller section having the divided rollers.
However, when the recording apparatus is so constructed as to conform any recording sheets even if the difference in size between the largest sheet and the smallest sheet is relatively large, it is desired that the distribution of the biasing force is strictly uniform with respect to all of the sizes of the sheets. Further, it has been highly requested that the recording sheet be conveyed with more and more accuracy, as the high density of the recording head has been progressed. For these reasons, it has been strongly requested that the distribution of the biasing force is more strictly uniformed with respect to all of the sized recording sheets. For example, an ink jet recording head which has recently been used as a recording or print head has been manufactured to have two hundred and fifty-six nozzles with dot density of 400 dots/inch (dpi). If such recording head is used, since the width of the head is 16.256 mm, it is necessary to maintain the accuracy of the feeding amount of the recording sheet on the order of 16.256 mm .+-.0.015 mm so as to make the junctions between recorded image portions imperceptible. Further, it has been necessary to provide strict or critical uniformity of the distribution of the biasing force for obtaining the above-mentioned accuracy on the order of 16.256 mm .+-.0.015 mm, when it is desired that the available width of the recording sheet be mated with all of the sizes thereof which differ relatively greatly, such as from A4 size (297 mm) to A2 size ( 594 mm).
However, in the earlier proposed (prior art) recording system shown in FIG. 9, the strict distribution of the biasing force cannot often be attained. For example, in FIG. 9, it is assumed that, by changing the positions at which the fixed plates 12a-12c are fixed, a biasing force of the biasing springs 4a-4c is adjusted to provide the uniform distribution of the biasing force for the A2 size recording sheet. FIG. 10A shows the relation between the conveying roller 1, intermediate roller 2, recording sheet 11 and biasing rollers 3a-3c, if the A4 size recording sheet is conveyed in such adjusted condition. In this case, since the intermediate roller 2 is not an absolute or true rigid body, as shown in FIG. 10A, the intermediate roller will be deformed in an area where the A4 size sheet does not exist, due to the fact that the portion of the intermediate in such area cannot resist against the biasing force F'.sub.B (FIG. 10B) of the biasing rollers 3b and 3c. Consequently, an end 11A of the recording sheet is subjected to concentrated stress. Thus, the distribution of the biasing force will be uneven as shown in FIG. 10B. Incidentally, in FIG. 10B, W'.sub.S represents the width of the recording sheet. If such uneven distribution of the biasing force is generated, as shown in FIG. 10C, when the recording sheet is conveyed after each scanning operation of the head unit 7, the recording sheet will hunt or meander each time, thus creating separation or overlap between the successively recorded image portions obtained by the successive scanning operations of the head unit 7. Incidentally, in FIG. 10C, W'.sub.R represents a width of the recorded image portion obtained by a single scanning operation of the head unit 7.
Thus, the present invention provides a sheet conveying apparatus which is improved to conform to the high accurate sheet conveyance which has been recently requested, and an image recording apparatus having such sheet conveying apparatus.